Problem 1877. Friday the 13th

According to superstition, lots of unpleasant stuff happens on Friday the 13th, so you might like to find the next occurence of such a date. Given a date string ('dd/mm/yyyy'), return the next occurence of a Friday the 13th. If the date string is already a Friday the 13th, return the original string. Output should be in the form of a date string ('dd/mm/yyyy').

Example: date = '13/09/2013', nextfri13(date) = '13/09'2013'; date = '15/12/2013', nextfri13(date) = '13/06/2014';

And so on. Good luck!